To improve the 17% 9-year cure rate obtained in earlier studies with combination chemotherapy for previously untreated children with acute lymphocytic leukemia. To determine in a controlled manner the relative efficacy and toxicity of two therapeutic regimens given early in remission for prophylaxis of central nervous system leukemia. To determine the efficacy and toxicity of periodic "reenforcement" or "reinduction" chemotherapy in prolonging continuous complete remission. To maintain the 50% 5-year cure rate obtained in studies with combination chemotherapy and preventive central nervous system therapy. To determine the optimal duration of continuation chemotherapy by comparing patients in "Total Therapy" programs who have received multiple-drug combination chemotherapy for 24, 28, 30 and 36 months.